Refrigerated vehicles have longed been employed in a wide variety of applications including the storing and marketing of perishable commodities, particularly produce such as fruit and vegetables, as well as other perishable foods, including processed and frozen or chilled products such as ice cream or the like. The refrigerated vehicles contemplated by the present invention include, for example, truck trailers for road transport and piggy-back use, railroad cars and container bodies contemplated for land and sea service and the like. Accordingly, all such refrigerated containers are included within the present invention under the general designation of refrigerated containers or vehicles adapted to receive cargo in a refrigerated space.
Refrigeration systems for such refrigerated containers have generally used conventional refrigerants in a closed loop system which included an evaporator for cooling the air in the container. Because of environmental concerns, hydrochloro flourocarbons (HCFCS), such as R-22 have been discontinued and hydroflourocarbons (HFCS), such as R-134a, R-410a and R-407c have taken their place. However, because of these same environmental concerns, it has been desirable to replace the use of these HFCS with a more “benign” refrigerant which does not adversely react with the atmosphere. Accordingly, the use of CO2 has now become attractive for use in vapor compression systems for refrigerated containers.
One characteristic of carbon dioxide as a refrigerant is that it has low critical temperature and therefore, most CO2 refrigerant vapor compression systems are designed for operation in the transcritical regime. This requires that they operate at substantially higher pressure then when operating with conventional refrigerants, and special compressors are designed for that purpose. In order to accommodate these higher pressures, the other components and tubing must be robust in their design. Still, the likelihood of leakage occurring within this system is greater than when operating with a lower pressure system with conventional refrigerants.
Depending on where in the system a leak may occur, the CO2 may be released to the outside atmosphere or to the inside of the container box. If released to the inside of the container box, the carbon dioxide concentration may create a potentially hazardous atmosphere to humans that may enter the container box. While the toxicity of CO2 is not likely to be a problem, the corresponding reduction in available oxygen may be harmful to one entering the container box. In this regard, OSHA has indicated that the lowest acceptable oxygen concentration for shift-long exposure is 19.5%, which corresponds to a carbon dioxide concentration of about 60,000 ppm (6%).
The use of carbon dioxide sensors within a refrigerant container has been made, but for a different purpose and manner. That is, in so called “controlled atmosphere” refrigerated container systems, the concentration of nitrogen within the container is enhanced so as to thereby decrease the oxidization (i.e. ripening) that may occur during shipping and storage. Such a system is described in U.S. Pat. No. 5,457,963. In such a system, there are certain types of cargo (e.g. asparagus, blueberry, blackberry, cantaloupe, fresh chili pepper) where the depletion of CO2 within the container is detrimental to the cargo. Thus, a known technique is to sense when the percentage of CO2 reaches a predetermined lower level and then responsively injecting CO2 into the container from a pressurized vessel that is provided for that purpose.
What is needed is a method and apparatus for determining when the content of CO2 has reached an undesirable higher level and providing notice thereof so that an operator does not enter the container under those conditions.